The present invention relates to antennas, specifically small printed antennas for low cost, short range wireless applications, for example in wireless toys, Wireless keyboards, wireless security systems, RF based remote controllers for TV sets, etc. At present, the relevant industry faces some major difficulties, particularly regarding: 1) miniaturization of antennas without a significant impact on performance; 2) lowering the cost of antennas and of antenna integration in the system; 3) the need for a low-loss filter attached to the antenna as part of the front-end rejection of out-of-band signals; and 4) the need for a low loss impedance matching network that will also maintain a stable matching, with minimal effect of production tolerances and/or of near human presence.
A traditional loop antenna is usually made to resonate when its physical length equals the electrical wavelength of the signal it receives or transmits. In the description below, a “loop” refers to any closed curve ending in a differential transmision line port. For applications that wish to use such an antenna, this puts a major limit on size and form factor. For example, a typical 2.4 Ghz antenna is about 12.5 cm in circumference, which is simply too large for many applications, for example for remote controllers, which require a smaller antenna. Reduction of antenna size in such applications is commonly done by dielectric loading. For example, the antenna can be embedded between layers of a ceramic substrate that has a high dielectric constant. As a consequence, the effective dielectric constant increases, which decreases the effective wavelength of the electrical signal at the antenna, and therefore decreases its size. However, dielectric loading significantly decreases antenna gain, as major parts of the transmitted or received energy dissipate in the dielectric material. This usually deviates the radiation pattern, and is also considered relatively expensive.
Various other methods to decrease the size of the antenna usually result in complicated and expensive matching networks. These methods usually use standard discrete matching components (capacitors, inductors), which have effects unwanted (such dominate especially in high frequencies, when the component dimensions become large with respect to the antenna dimensions or wavelength). In many cases, a lot of energy is wasted on these components, so the antenna gain is decreased. Such components usually have production tolerances. That means, for example, that they cannot be used to create a narrow-band antenna—as the central frequency of the impedance matching will vary from one device to another.
The issue of filtering is also very important. The short range, wireless applications industry usually requires filtering at the antenna port, to protect the system from interfering signals, and to prevent radiation of out-of-band signals, in order to comply with electromagnetic compatibility regulations, such as FCC regulations in the USA. Therefore, expensive filters are installed at the antenna port. Common filters usually suffer from ‘insertion loss’, which means that they also block some of the energy that is within the required frequency band.
Integration of the antenna with an RF amplifier also raises several problems. RF chips usually show a high output impedance, in order to have a low current consumption. Most antennas in the market today, are built to match the traditional 50 ohm impedance, which again, requires use of another lossy and expensive matching network in between. In addition, a balanced interface also improves the power efficiency of RF chips, so many chip manufacturers today design chips that have a balanced (differential) RF port. As most antennas in the market today are created unbalanced, which means that they are fed with a non-differential transmission line such as a microstrip line, a “balun” component is required at the antenna chip interface, which also adds to the cost and to the energy losses.
Another difficulty that exists, is the fact that human presence near the antenna affects the performance of the antenna. That is due to the fact that the high dielectric constant of the human tissue ‘absorbs’ the electric fields that are produced by the antenna. An alternative way to describe it is to say that the human tissue behaves as a load that is coupled to the antenna and therefore changes the input impedance that is measured at the antenna port. Since many applications today are ‘hand-held’ applications, there are major difficulties to maintain a matching to the antenna that is not affected by the human presence. This, sometimes, forces radio designers to increase transmission power by several orders of magnitude (as happens in portable phones).
There is thus a widely recognized need for, and it would be highly advantageous to have, a high performance small antenna that is matched to a required output impedance, does not require filtering, is simple and inexpensive to manufacture, and is easily integrable to an RF power amplifier—with minimum cost, minimum external components and minimum energy losses. The present invention overcomes all the difficulties listed above, and provides these advantages by a novel method to design antennas, with examples showing antennas designed using this method.